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sustained RPM's

Discussion in '1982-Present GM Diesel' started by 55Willy, Mar 30, 2003.

  1. 55Willy

    55Willy 3/4 ton status

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    sustained RPM\'s

    I'm going to cali on the 11th of April, about a 5 hour drive, most of the way (90%) will be at 75 MPH. with 4.10's and 35's (th400) Only stopping for fuel. Can I get good mileage doing this or should I think more along the lines of 60 or 65???


    -Jeremy
     
  2. azblazor

    azblazor 1/2 ton status Premium Member

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    Re: sustained RPM\'s

    If you own a GM diesel I highly recommend joining at http://www.thedieselpage.com/

    Excerpted for "The Diesel Page" public area:

    The "GM Product Service Training Manual for the 6.2L Diesel Engine" (#16015.05-1D) has this to say about GM diesel fuel mileage:

    "The diesel, like any engine, is affected by driving habits. Speed is more critical on a diesel than a gas engine. On the highway, in the 50-75 mph range, the fuel economy will go down about 3 mpg for each 10 mph increase in speed. A gasoline engine will lose about 1-1/2 mpg for each 10 mph increase in speed. This condition is perhaps the most significant factor in obtaining good fuel ecomomy. Fuel economy may vary as much as 5 mpg in a given vehicle with different drivers."

    Diesel engines have about half the useable rpm range as compared to a gas engine. A gasoline engine will produce acceptable fuel economy over a fairly wide rpm range. The diesel engine has a relatively narrow rpm range that will produce acceptable fuel economy.

    The relationship of gearing to fuel mileage has to be the single most misunderstood aspect of the GM diesel engine. A typical 6.5TD with 3.08 or 3.42 gearing will get in excess of 20 mpg at 65 mph because the engine rpm at that speed is right at the engine's torque peak of about 1700-1800 rpm. A truck with 4.10 gearing is running at about 2250 rpm at the same speed and typically gets 15 mpg. A 500 rpm difference might not sound like much, but in a diesel, that's about half of the useable rpm range.

    6.5TD engine rpm (4L80E transmission) and typical fuel mileage at 65 mph:
    3.08 gearing 1690 rpm (23 mpg)
    3.42 gearing 1880 rpm (21 mpg)
    3.73 gearing 2050 rpm (17 mpg)
    4.10 gearing 2250 rpm (15 mpg)

    If you're into high speed interstate type driving, the GM diesel will deliver the performance you're after as long as you gear your truck for the speed you want to drive. The 3.08 and 3.42 gears will allow you to drive at any sane speed. The 3.73 and 4.10 gears are meant to tow at a lower speed. It's that simple.

    If you're buying a new diesel truck and want the best possible fuel mileage, get a truck with the highest gear ratio available and an overdrive transmission. Specify a 3.42 gear ratio unless you intend to use the truck to tow. 1998+ axle ratio options for diesel engine equipped C/K trucks include 3.08, 3.42, 3.73, 4.10, and 5.13 gearing is offered in some heavy duty model GMC 6.5TD trucks . The 3.08 ratio is available only for the C1500 series versions (2WD) and the K2500 series Suburbans are only available with the 3.73 or 4.10 axle ratios. The new 4L80-E overdrive automatic or a 5 speed overdrive manual transmissions are available in all diesel trucks.

    General differential ratio recommendations for towing with the 6.5TD:
    Up to 4,000 lb trailers: 3.42
    4,000 to 6,000 lb trailers: 3.73
    6,000 to 8,000 lb trailers: 4.10
     
  3. Txstruck

    Txstruck Registered Member

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    Re: sustained RPM\'s

    I have an 86 K30 4x4 with a 6.2/TH400 and 4.56 gears and I drove from Dallas to Austin non stop and it used just over a half a tank of fuel. I only drove 55mph since I have low gears and 32" tires but the rpm's were definately humming. And it took just over 4 hrs. /forums/images/graemlins/crazy.gif /forums/images/graemlins/truck.gif
     
  4. arveetek

    arveetek 1/2 ton status

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    Re: sustained RPM\'s

    Good advice.

    However, the only thing that the information given doesn't talk about, is tire size. All that info is based on stock tire size. I would imagine the stock tires on the trucks are closer to 27-30". 35" are much larger, and therefore the rpms will be lower. However, there is more mass to turn, and the wind resistance is much higher at 70 + than at 60-65mph. So it probably doesn't make that much difference. Just my observations!

    I'd say drive as fast as you want, and if it seems like the fuel mileage isn't all that great, slow down to see if it helps. I know that on my truck with 4.10 gears, OD transmission, and 27" tires, I get 18.5 mpg at 55 mph or 70 mph....doesn't seem to make much difference.

    Casey
     
  5. doctor4x4

    doctor4x4 1/2 ton status

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    Re: sustained RPM\'s

    33" A.T. bf goodrich and 6.2 banks Turbo 700R4-208 combined city and freeway = 16 mpg 4 me average and freeway only about 22 mpg at 70 - 75 mph
    1984 Burb checking in /forums/images/graemlins/burb.gif /forums/images/graemlins/usaflag.gif
     

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